Factors Modulating the Expression of Biogenic Amine Receptors in the Honeybee Brain and Antennae: Possible Implications for Memory and Learning

The biogenic amines dopamine (DA) and octopamine (OA) are known to play important roles in modulating olfactory learning in insects. In the honeybee, levels of these amines increase with age, as does aversive learning performance. DA and OA signalling is mediated by G-protein coupled trans-membrane receptors. A major goal of this thesis was to examine the relationship between shifts in the expression of a suite of DA and OA receptor genes and levels of aversive learning in honeybee workers. A growing body of evidence indicates that the actions of the biogenic amines are not restricted to the central nervous system. Therefore, the second major goal of this thesis was to examine the potential relationship between changes in worker bee behaviour and shifts in the peripheral (antennal) expression of amine receptor genes. Using bees collected at various ages between emergence and when bees normally perform foraging duties, age-related shifts in the whole brain expression of the DA receptor genes Amdop1, Amdop2, Amdop3, the putative DA/ecdysone receptor Amgpcr19 and an OA receptor Amoa1, were examined using quantitative real time PCR. The results indicate that the brain expression of amine receptor genes shows age-related variation. Using laser capture microdissection it was also possible to examine shifts in gene expression in subpopulations of MB neurons (Kenyon cells). Using this technique it was demonstrated that the expression of amine receptor genes exhibited both temporal and spatial plasticity. These results suggest a functional subdivision of the subpopulations of MB neurons, consistent with evidence from Drosophila indicating that different MB subpopulations play different roles in the formation and retrieval of olfactory memories. Experiments were then conducted to examine the relationship between shifts in amine receptor gene expression and aversive learning performance. Factors that might drive age-related changes in the expression of DA and OA receptor genes were examined during the first week of adult worker bee life. These factors included juvenile hormone (JH), 20-hydroxyecdysone (20-E) and queen mandibular pheromone (QMP). While each of these factors induced shifts in receptor gene expression, JH, which is known to pace behavioural development, produced the most striking results. JH was seen to induce Kenyon cell specific shifts in the expression of DA and OA receptors that correlated with the induction of learning in 2-day old bees reared with QMP that did not otherwise show an ability to learn. These results indicate that learning and memory are likely to be modulated by a complex interplay between DA and OA signalling in sub-populations of MB neurons and that the effects of DA and OA upon the formation and retrieval of olfactory memories will depend upon the complement of receptors expressed in specific Kenyon cell populations. The antennal expression of amine receptor genes also showed changes associated with age and with exposure to QMP, JH and 20-E. In addition, higher expression levels of the OA receptor gene Amoa1 correlated with worker attraction to QMP. The behavioural consequences of all of the observed shifts in expression are not fully understood. However, colony manipulations used to induce precocious foraging showed that a reduction in the expression of Amoa1 can be behaviourally driven. JH treatment in 2-day old QMP reared bees caused a similar reduction that was shown to be associated with reduced worker attraction to QMP. These results strongly suggest that peripheral actions of the biogenic amines are likely to play a functional role in modulating worker bee behaviour. Demonstration of direct correlations between either the central versus peripheral shifts in receptor gene expression and changes in aversive learning performance was beyond the scope of this thesis. However, the results presented strongly suggest that future models of memory and learning should consider the influence of biogenic amines at multiple neural levels